RADARSAT-2 observations of SAR data near/around the island of Guam in the Marianas Islands continue in November (as previously discussed here, here and here). Overpasses on 15 November (above) and 17 November (below) contain features that deserve comment. For example, there is an obvious seam (and a less-obvious second seam) in the SAR wind analysis above. The “Beam Seams” arise because the 500-km wide scan for SAR winds is created from smaller individual beam positions.   Each beam position has its own set of adjustments applied during SAR image processing and sometimes there is slight disagreement from one beam to the next.   The image above has 3 subswaths – so 2 beam seams. The seams are also apparent in the normalized radar cross section (NRCS) image here.

Although SAR winds as displayed in AWIPS do not have a wind direction, a direction can be inferred from the ‘shadow’ of weaker winds in the lee of Guam. Imagery at this website does include the constraining GFS wind barbs (as shown here). Note that MetopB Advanced Scatterometer (ASCAT) winds show winds (from this site) in the 15-20 knot range in the region to the west of Guam, in agreement with the SAR observations.

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SAR winds on 17 November at 0830 UTC, shown below with Himawari-8 infrared imagery, show winds of similar strength to those on the 15th. (Click here to view the analysis with wind barbs, or the NRCS analysis, both from this site.) On this day, the Marianas Islands all have an apparent effect on the downstream winds (Click here to see the AWIPS image of SAR winds; here’s a similar example over the Caribbean). The wake from Rota, for example, extends all the way from Rota to the western edge of the SAR scan! The wake from Guam is being affected by convection, but it too can be followed to the western edge of the SAR domain. Islands in the Marianas have an effect on winds that stretches for 100s of kilometers on some days, such as the 17th, but not on others, such as the 15th. (Note also: very faint Beam Seams are detectable on the 17th).

This SAR scan includes the islands of Tinian and Saipan, as shown below. Are there still large ships near Saipan, as noted in this blog post where 5 are present? Per the SAR data below, perhaps only one! Also, the lighter winds downwind of both Saipan and Tinian are very apparent in the image below: winds of 20 knots on the windward side of the islands (green in the enhancement used) and 10 knots or less (blue and purple) on the leeward side.

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You can find information on RADARSAT-2 at this WMO link.

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Overlapping 1-minute Mesoscale Domain Sectors provided 30-second images from GOES-16 (GOES-East) ABI spectral bands 01-16, in addition to a Rocket Plume RGB (above) — which displayed visible reflectance and/or thermal signatures of the NASA Artemis Space Launch System rocket booster (as it moved quickly northeastward) and its low-altitude launch condensation cloud (as it drifted slowly eastward away from the coast) on 16 November 2022. One or both of these signatures was seen in all 16 of the ABI spectral bands, as well as the RGB imagery.

A closer view of the area immediately offshore of the Kennedy Space Center launch site (below) showed the faint visible reflectance signature in Band 01, along with the brighter signature in Band 02.

GOES-16 Rocket Plume RGB images created using Geo2Grid are shown below.

GOES-18 (GOES-West) caught a glimpse of the rocket booster’s thermal signature (shades of green) far to the east of Florida during its Full Disk scan that began at 0650 UTC (below).

Thermal signatures of the rocket booster (brighter shades of pink) and its low-altitude condensation cloud (darker shades of blue) were also apparent in GOES-16 Nighttime Microphysics RGB images from the CSPP GeoSphere site (below).

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The animation above shows the Nighttime Microphysics RGB (saved from the CSPP Geosphere site) over the islands of American Samoa. (An animated gif is available here). The annotated animation below highlights several features that are typical of convective development detection at night. Low clouds become apparent shortly before 1450 UTC (when they are circled); subsequent color changes in those low clouds (in this case, a reddening that then detaches from the cloud) are most likely orphaned anvils (highlighted with arrows in the 1500-1530 UTC imagery — but the orphaned anvil can be tracked at later times, as it moves farther to the south) whose appearance frequently precedes successful convection; further reddening of the low clouds, as highlighted by arrows from 1610 to 1630 UTC, shows a transition of the low clouds (made up of water droplets) to higher clouds that are glaciating. At the end of the animation, note that active convection is near the Manu’a Islands.

Careful inspection of the Night Microphysics RGB can help you anticipate where convection might be developing at night.

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On 14 November 2022, American Samoa (indeed, all the Samoan Islands) were withing deep moisture associated with the South Pacific Convergence Zone, a common occurrence. The animation of MIMIC Total Precipitable Water, above, shows the evolution of the moisture hourly from 0000 to 1200 UTC on 14 November. (MIMIC fields in various domains are available in real time at this website; archived imagery is here). Note that 1200 UTC is 1 AM in American Samoa.

A forecaster in the morning might look at GDI (Gálvez-Davison Index, also discussed on this blog here) that is created from the GFS, as available at this NCEP link. (one can also search on ‘GDI NCEP’). The forecast fields from the 1200 UTC/14 November run are shown below (courtesy Jose Galvez and Bonnie Acosta, NOAA). The GDI shows an increase in values over Samoa first, then those large values overspread Tutuila (the largest of the islands of American Samoa) by 0000 UTC; by 0600 UTC, the back edge of the larger values is approaching Tutuila from the west. (For reference, Click here to see a map of the Samoan Islands; from west to east the main islands are Savai’i and Upola (the country of Samoa), then Tutuila, Ofu, Olosega and Ta’u, the four main islands of American Samoa).

Bearing the GDI fields above in mind, consider the evolution of Band 13 imagery shown below from GOES-18. Deep convection is persistent to the south of the Samoan Islands in a band that is roughly aligned with the largest forecast GDI values at 1200 and 1800 UTC. A region of stronger convection with an axis south-southwest to north-northeast develops across Savai’i and Upola. This development in the infrared imagery is very much in line with the forecast evolution of GDI fields that shows increasing values of the GDI in a line stretching southwest to northeast of Samoa.

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The evolution of the Band 13 imagery from 2110 UTC to 0610 UTC is also consistent with the GDI predictions — a back edge to the convection is approaching Tutuila by 0610 UTC. This edge is apparent in the infrared imagery, but also in the MIMIC Total Precipitable water (here is the 0600 UTC image).

The Weather Office in Pago Pago issued a Flood Watch at around 4 PM Samoa Time on 14 November (that is, around 0300 UTC on 15 November). Trends in the satellite imagery, and in total Precipitable Water fields, and in the GDI forecast were all used to help decide on whether extra staffing would be needed after sunset to deal with flooding-related work. (Extra staff were not called in).

GDI forecasts and satellite-based trends in infrared imagery and microwave estimates of total precipitable water can help with staffing decisions at forecast offices.

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